U.S. patent application number 11/709175 was filed with the patent office on 2007-08-23 for motorcycle exhaust structure.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Eiji Adachi, Yuji Noguchi.
Application Number | 20070193812 11/709175 |
Document ID | / |
Family ID | 38042729 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070193812 |
Kind Code |
A1 |
Adachi; Eiji ; et
al. |
August 23, 2007 |
Motorcycle exhaust structure
Abstract
To place a muffler in a front area to make it suitable for an
off-road motorcycle, A down frame extending downward obliquely from
a head pipe ahead of an engine and a lower frame connected with its
lower end, extending backward under the engine, are provided. A
muffler is located in the vicinity of the joint between the down
frame and the lower frame where its rear surface is curved forward
in a convex pattern along the down frame and the lower frame. An
exhaust pipe extending from a cylinder head is almost S-shaped as
seen sideways and its forward curved portion and lower backward
curved portion are vertically arranged with its lower end connected
with a lower portion of the muffler.
Inventors: |
Adachi; Eiji; (Saitama,
JP) ; Noguchi; Yuji; (Saitama, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
38042729 |
Appl. No.: |
11/709175 |
Filed: |
February 22, 2007 |
Current U.S.
Class: |
180/309 |
Current CPC
Class: |
F01N 2590/04 20130101;
F01N 13/08 20130101; F01N 2340/04 20130101 |
Class at
Publication: |
180/309 |
International
Class: |
B60K 13/04 20060101
B60K013/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2006 |
JP |
2006-047582 |
Claims
1. An exhaust structure for a motorcycle, the motorcycle including
a body frame supporting a front wheel and an engine, the engine
including a crankcase and a cylinder extending upward from the
crankcase, said exhaust structure comprising: an exhaust pipe
extending from the cylinder of the engine; and a muffler, the
exhaust pipe being connected with the muffler, wherein the muffler
is located ahead of the crankcase and behind the front wheel when a
front suspension of the motorcycle is in a retracted position.
2. The exhaust structure according to claim 1, wherein the muffler
is located above a lower end of a body frame.
3. The exhaust structure according to claim 1, wherein the body
frame has a front frame extending downward from the head pipe and
extending downward from ahead of the engine and a rear surface of
the muffler is curved along the front frame.
4. The exhaust structure according to claim 2, wherein the body
frame has a front frame extending downward from the head pipe and
extending downward from ahead of the engine and a rear surface of
the muffler is curved along the front frame.
5. The exhaust structure according to claim 1, wherein a high front
fender is located above the front wheel and the muffler is located
under a rear end of the front fender.
6. The exhaust structure according to claim 2, wherein a high front
fender is located above the front wheel and the muffler is located
under a rear end of the front fender.
7. The exhaust structure according to claim 1, wherein the exhaust
pipe is connected with an upper front of the cylinder and the
exhaust pipe includes an upper forward curved portion and a lower
backward curved portion and the backward curved portion is
connected with a rear lower portion of the muffler.
8. The exhaust structure according to claim 2, wherein the exhaust
pipe is connected with an upper front of the cylinder and the
exhaust pipe includes an upper forward curved portion and a lower
backward curved portion and the backward curved portion is
connected with a rear lower portion of the muffler.
9. The exhaust structure according to claim 1, wherein the exhaust
pipe is connected with an upper front of the cylinder and the
exhaust pipe includes a side curved portion extending downward
while curving on one side of a vehicle body, and a horizontally
extended portion extending from a lower end of the side curved
portion ahead of the crankcase in a transverse direction of the
body and bending in an opposite direction, and an exhaust
downstream end of the horizontally extended portion is connected
with the inside of the muffler.
10. The exhaust structure according to claim 1, wherein the muffler
is located to cover an area in front of a vicinity of a joint
between a down frame and a lower frame of the body frame.
11. A motorcycle, comprising: a body frame with a head pipe
supporting a front wheel; an engine supported by the body frame,
said engine including: a crankcase in a lower position of the
engine; and a cylinder extending upward from the crankcase; an
exhaust pipe extending from the cylinder; and a muffler, the
exhaust pipe being connected with the muffler, wherein the muffler
is located ahead of the crankcase and behind the front wheel when a
front suspension of the motorcycle is in a retracted position.
12. The motorcycle according to claim 11, wherein the muffler is
located above a lower end of the body frame.
13. The motorcycle according to claim 11, wherein the body frame
has a front frame extending downward from the head pipe and
extending downward from ahead of the engine and a rear surface of
the muffler is curved along the front frame.
14. The motorcycle according to claim 12, wherein the body frame
has a front frame extending downward from the head pipe and
extending downward from ahead of the engine and a rear surface of
the muffler is curved along the front frame.
15. The motorcycle according to claim 11, wherein a high front
fender is located above the front wheel and the muffler is located
under a rear end of the front fender.
16. The motorcycle according to claim 12, wherein a high front
fender is located above the front wheel and the muffler is located
under a rear end of the front fender.
17. The motorcycle according to claim 11, wherein the exhaust pipe
is connected with an upper front of the cylinder and the exhaust
pipe includes an upper forward curved portion and a lower backward
curved portion and the backward curved portion is connected with a
rear lower portion of the muffler.
18. The motorcycle according to claim 12, wherein the exhaust pipe
is connected with an upper front of the cylinder and the exhaust
pipe includes an upper forward curved portion and a lower backward
curved portion and the backward curved portion is connected with a
rear lower portion of the muffler.
19. The motorcycle according to claim 11, wherein the exhaust pipe
is connected with an upper front of the cylinder and the exhaust
pipe includes a side curved portion extending downward while
curving on one side of a vehicle body, and a horizontally extended
portion extending from a lower end of the side curved portion ahead
of the crankcase in a transverse direction of the body and bending
in an opposite direction, and an exhaust downstream end of the
horizontally extended portion is connected with the inside of the
muffler.
20. The motorcycle according to claim 11, wherein the muffler is
located to cover an area in front of a vicinity of a joint between
a down frame and a lower frame of the body frame.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This nonprovisional application claims priority under 35
U.S.C. .sctn. 119(a) on Patent Application No. 2006-047582, filed
in Japan on Feb. 23, 2006, the entirety of which is incorporated
herein by reference.
[0002] 1. Field of the Invention:
[0003] The present invention relates to a motorcycle exhaust
structure. In particular, the present invention relates to a
motorcycle exhaust structure that is suitable for an off-road
vehicle.
[0004] 2. Description of Background Art:
[0005] A motorcycle exhaust structure is known, wherein a main
frame extending downward obliquely and backward from a head pipe
supporting a front wheel is provided. A V-type engine is held under
the main frame. An exhaust pipe extending from a cylinder of the
engine is connected to a muffler divided into two portions before
and after an oil pan under a crankcase. The rear muffler portion is
larger than the front muffler portion (see JP-A No.
2004-26007).
SUMMARY OF THE INVENTION
[0006] In a situation where a muffler like the above known one is
located under a crankcase, a relatively large and heavy muffler may
be located in a lower portion of a vehicle body. This arrangement
is suitable for an on-road motorcycle, which is requested to be
able to move its body agilely by placing a heavy component in a
position as low as possible in the body, thus reducing the inertia
force when inclining the body left or right.
[0007] However, in an off-road motorcycle, since its cylinder
should be upright and the minimum road clearance should be high in
order to ensure that the engine is longitudinally short, it is
impossible to place a muffler under a crankcase. Therefore, this
application has an object to make it possible that a muffler is
located near a head pipe even in such an off- road motorcycle.
[0008] In order to address the above problem, a first aspect of the
present invention is directed to a motorcycle exhaust structure,
wherein a motorcycle includes a body frame with a head pipe
supporting a front wheel that supports an engine. The engine
includes a crankcase in a lower position and a cylinder extending
upward from the crankcase. An exhaust pipe extending from the
cylinder is connected with a muffler. The muffler is located ahead
of the crankcase and behind the front wheel with a front suspension
in its retracted position and above a lower end of the body
frame.
[0009] According to a second aspect of the present invention, the
body frame has a front frame extending downward from the head pipe
and extending downward from ahead of the engine and a rear surface
of the muffler is curved along the front frame.
[0010] According to a third aspect of the present invention, a high
front fender is located above the front wheel and the muffler is
located under a rear end of the front fender.
[0011] According to a fourth aspect of the present invention, the
exhaust pipe is connected with an upper front of the cylinder and
the exhaust pipe includes an upper forward curved portion and a
lower backward curved portion and the backward curved portion is
connected with a rear lower portion of the muffler.
[0012] According to a fifth aspect of the present invention, the
exhaust pipe is connected with an upper front of the cylinder and
the exhaust pipe includes a side curved portion extending down
while curving on one side of the body. A horizontally extended
portion extends from a lower end of the side curved portion ahead
of the crankcase in a transverse direction of a body and bending in
an opposite direction. An exhaust downstream end of the
horizontally extended portion is connected with the inside of the
muffler.
[0013] According to the first aspect of the present invention, the
muffler is located ahead of the crankcase and behind the front
wheel with the front suspension in its retracted position and above
a lower end of the body frame. Therefore, the muffler can be near
the head pipe. Therefore, when the vehicle body moves centering
around the head pipe, the inertia force is reduced. Moreover, the
minimum road clearance can be high, which is suitable for an
off-road motorcycle.
[0014] In addition, when the front and rear of the vehicle body
move vertically on an uneven road surface, since the muffler is
near the engine, heavy components are concentrated in the vicinity
of the center of the body. As a consequence, the inertia force is
reduced, thereby decreasing vertical movement of the front and rear
of the body.
[0015] According to the second aspect of the present invention, a
rear surface of the muffler is curved along the front frame
extending downward from the head pipe and extending downward from
ahead of the engine. Therefore, despite the presence of the front
frame, the muffler can be located ahead of the crankcase and near
the front frame, permitting the engine to lie in a more forward
position.
[0016] According to the third aspect of the present invention, a
high front fender is located above the front wheel and the muffler
is located under a rear end of the front fender. Therefore, the
muffler can be in a position not to interfere with the front
fender.
[0017] According to the fourth aspect of the present invention, the
exhaust pipe extending from the upper front of the cylinder
includes an upper forward curved portion and a lower backward
curved portion and the backward curved portion is connected with
the rear lower portion of the muffler. Therefore, a sufficiently
long exhaust pipe length is ensured.
[0018] According to the fifth aspect of the present invention, the
exhaust pipe includes a side curved portion extending downward
while curving on one side of the body, and a horizontally extended
portion extending from the lower end of the side curved portion
ahead of the crankcase in a transverse direction of the body and
bending back, and the exhaust downstream end of the horizontally
extended portion is connected with the muffler. Therefore, it is
ensured that the exhaust pipe is sufficiently long and can be
arranged in a compact manner by taking advantage of the inside of
the large-capacity muffler extending transversely.
[0019] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
[0021] FIG. 1 is a side view of a motorcycle according to an
embodiment;
[0022] FIG. 2 is an enlarged side view mainly showing a
muffler;
[0023] FIG. 3 is a view of a body frame and an exhaust system as
seen from the direction of arrow B in FIG. 2;
[0024] FIG. 4 is a fragmentary sectional view showing the
positional relation between the muffler and the front frame;
[0025] FIG. 5 is a plan view of the exhaust system;
[0026] FIG. 6 is a front view of the above including a partial
cutaway view; and
[0027] FIG. 7 is a sectional view taken along the line 7-7 in FIG.
5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] An embodiment will now be described with reference to the
accompanying drawings, wherein the same or similar elements will be
identified with the same reference numeral throughout the several
views.
[0029] FIG. 1 is a side view of an off-road motorcycle to which a
body frame structure according to this embodiment is applied. The
body frame 1 of this motorcycle includes a head pipe 2, a main
frame 3, a center frame 4, a down frame 5 and a lower frame 6,
which are connected in a loop-like manner with an engine 7 held
inside the loop. The down frame 5 and the lower frame 6 constitute
a front frame in this application. The constituent parts of the
body frame 1 are each made of appropriate metal materials such as
aluminum alloy. The head pipe 2 and the center frame 4 are
castings. The main frame 3 and the lower frame 6 are square pipe
wrought products made by extrusion molding. The down frame 5 is a
combination of a wrought product and a casting.
[0030] The main frame 3 extends straight downward obliquely and
backward above the engine 7 and joins the upper end of the center
frame 4 extending vertically behind the engine 7. The down frame 5
extends downward obliquely ahead of the engine 7 with its lower end
connected to the front end of the lower frame 6. The lower frame 6
curves from the front lower portion of the engine 7 to an area
under the engine 7 and then extends almost straight backward with
its rear end connected to the lower end of the center frame 4.
[0031] The engine 7 is a water-cooled 4-cycle engine with a
cylinder 8 and a crankcase 9. The cylinder 8, located on the front
of the crankcase in an upright position with its cylinder axis line
almost vertical, includes a cylinder block 10, a cylinder head 11
and a head cover 12.
[0032] A fuel tank 13 is located above the engine 7 and is held on
the main frame 3. A seat 14 is located behind the fuel tank 13. The
seat 14 is held on a seat rail 15 that extends backward from the
upper end of the center frame 4. Reference numeral 16 represents a
reinforcing pipe located under the seat rail 15. The seat rail 15
and the reinforcing pipe 16 support an air cleaner 17. Air is taken
into the cylinder head 11 through a throttle body 18 from behind
the body.
[0033] An exhaust pipe 20, extending downward from the front of the
cylinder head 11 ahead of the engine 7, is connected to the muffler
21 located in front of the crankcase 9. A tail pipe 22 extends
backward from the muffler 21, crossing the center frame 4 behind
which its rear end is supported by the reinforcing pipe 16.
[0034] The head pipe 2 supports a telescopic front fork 23,
constituting a front suspension. A front wheel 24 supported by its
lower end is steered by a handlebar 25. The center frame 4
swingably supports the front end of a rear arm 27 through a pivot
shaft 26. A rear wheel 28 is supported by the rear end of the rear
arm 27 and is driven through a chain by the engine 7. A cushion
unit 29 as a rear suspension is provided between the rear arm 27
and the rear end of the center frame 4.
[0035] Reference numeral 30 represents a radiator; 31 a rubber
mounting part for the radiator; 32 and 33 engine mounting parts; 34
an engine hanger; and 35 an electric component case. The center
frame 4 also supports the engine 7 through the pivot shaft 26.
[0036] FIG. 2 is an enlarged side view showing the muffler 21 and
its vicinity. The exhaust pipe 20 is curved almost in an S pattern
as seen sideways where its upper forward-curved portion 40
protrudes forward from the cylinder head 11 and curves forward in a
convex pattern while its lower backward-curved portion 41 curves
backward in a convex pattern with its lower end connected with the
muffler 21. The muffler 21 is held on the lower frame 6 by its stay
42 extending backward from its back bottom being fitted to a stay
6b of a front-end 6a of the lower frame 6. The front end 6a of the
lower frame 6 is a portion curving upward so that that the lower
frame 6 runs under the crankcase 9 and turns around it to come in
front of it.
[0037] The muffler 21 is located in a way to cover an area in front
of the vicinity of the joint between the lower end of the down
frame 5 and the lower frame 6. The muffler 21, virtually in the
form of a triangle with its front middle part convex forward as
seen sideways, is located in a position and a form not to interfere
with the front wheel 24 in its higher limit position (indicated by
virtual line A) with the front fork 23 in its most retracted
position.
[0038] The muffler 21 is behind a vertical line V passing through
point P as the rearmost end of the front wheel 24 in its position
as indicated by the virtual line A. It is also under the respective
vicinities of point Q as the rear end of the high front fender 24a,
and point P, and the points themselves. Also it is located ahead of
point R as the bottom of the lower frame 6 and slightly above a
horizontal line L passing through point R. Point R is also the
point of minimum road clearance of the body frame 1 as a whole.
[0039] FIG. 3 is a front view of the body frame 1 and the muffler
21. The muffler 21, lying horizontally long so as to be wide
transversely (in the left-right direction), is a relatively
large-capacity component extending left and right, which includes
an inlet cap 43, a main portion 44 and an outlet cap 45. The inlet
cap 43 and the outlet cap 45 each virtually take the form of a
funnel protruding upward and the inlet cap 43 is fitted to the main
portion 44 lying on vehicle body centerline C, covering an opening
from left and the outlet cap 45 is fitted from right.
[0040] The exhaust pipe 20 extends downward curving in a way to
protrude left outward from the vicinity of the vehicle body
centerline C and joins an inlet 43a at the rearmost end of the
inlet cap 43. The tail pipe 22 extends upward obliquely and
backward on the right side of the vehicle body from an outlet 45a
at the highest end of the outlet cap 45.
[0041] In the body frame 1, the head pipe 2 and the down frame 5
are each singly arranged vertically along the vehicle body
centerline C. Left and right main frames 3, center frames 4 and
lower frames 6 are arranged in pairs. The left and right ends of
the muffler 21 protrude more outward than the left and right lower
frames 6 and the left and right center frames 4 where points S as
their outer bottom points determine banking angle q.
[0042] The points S which determine the banking angle for the inlet
cap 43 and outlet cap 45 on the left and right are transversely
symmetrical, taking care that the left and right banking angles q
are equal. In this front view, the lower ends of the lower frames 6
are visible under the lower end of the muffler 21.
[0043] The exhaust pipe 20 is connected with the inlet 43a at the
back of the rear lower portion of the inlet cap 43 located in a
left lower portion of the muffler 21. The tail pipe 22 extends
upward obliquely and backward on the right side of the vehicle body
from the outlet 45a as the upper protruding end of the outlet cap
45 provided in a right upper portion of the muffler 21 and once
gets right of the center frame 4, then curves back inward in the
vehicle body and extends backward between the down frame 5 and the
center frame 4.
[0044] The down frame 5 singly extends downward from the head pipe
2 along the vehicle body centerline C and stiffeners 36 as left and
right branches of its upper portion extend like a bowl. Integrally
formed expanded main joints 36a at their tips abut on the bottom
surfaces of the paired left and right main frames 3 respectively
and their peripheries are welded. The rear end of the tail pipe 22
runs under the stiffeners 36 through a space surrounded by the main
frames 3, center frames 4 and down frame 5, extending backward.
[0045] The lower end of the down frame 5 has a lower joint 37 as a
point of bifurcation and a pair of left and right branches 37a
integrally formed with the lower joint 37 are connected with the
respective front ends 6a of the paired left and right lower frames
6 and welded together. A stay 46, provided on the upper surface of
the main portion 44 of the muffler 21, is fixed with a bolt 46a on
a front surface of the lower joint 37.
[0046] FIG. 4 is a left side view mainly showing an exhaust system
including the exhaust pipe 20, muffler 21 and tail pipe 22, where a
cutaway section of the muffler 21 is shown in order to illustrate
the relation between the muffler 21 and the body frame 1. As
apparent from this figure, the rear wall 47 of the muffler 21
curves forward in a convex pattern, extending vertically in front
of the down frame 5 and the front end 6a of the lower frame 6,
keeping a small clearance from them.
[0047] The front end of the lower frame 6 extends upward along the
curved surface of a front lower surface of the crankcase 9, curving
forward in a convex pattern and joins the branches 37a at the lower
joint 37 of the down frame 5. Thus, the rear wall 47 of the muffler
21 runs from ahead of the lower joint 37 along an area ahead of the
front end 6a of the lower frame 6 under the joint with the branches
37a and in its vicinity. This makes it possible to decrease the
amount of forward protrusion of the muffler 21 from the crankcase
9. In the figure, reference numeral 37c represents a boss for
mounting the stay 46 formed on a surface of the lower joint 37.
[0048] FIG. 5 is a plan view of the exhaust system where the main
portion 44 takes the form of a hollow cylinder that opens left and
right with the inlet cap 43 and the outlet cap 45 welded integrally
at both the left and right sides thereof. The inlet cap 43 and the
outlet cap 45 each consist of a front half and a rear half. After
the front and rear halves are joined to make a hollow cap, the
joint is welded. Reference numeral 22a represents a bracket for
fitting the rear end 22a of the tail pipe 22 to the auxiliary pipe
16 (FIG. 1).
[0049] FIG. 6 is a view of the exhaust system as seen from the
direction of arrow B in FIG. 2 including a partial cutaway view of
the muffler 21, where the exhaust pipe 20 enters, as an extension
20a, the inlet cap 43 through the inlet 43a and extends
horizontally toward the main portion 44 on the bottom of the inlet
cap 43 and, inside the main portion 44, joins a front portion 48a
of a bend 48.
[0050] The bend 48 is a pipe member including a front portion 48a
and a rear portion 48c that are horizontal and vertically parallel
to each other, and a middle portion 48b that connects them
vertically. The front portion 48a lies horizontally on the main
portion 44 and the bottom of the joint of the outlet cap 45. The
middle portion 48b is in an upright position inside the outlet cap
45 extending from the bottom to the top with its upper end
continuous with the rear portion 48c. The rear portion 48c lies
horizontally from the outlet cap 45 along the line from the joint
of the main portion 44 to the top where its downstream end 48d is
open in the vicinity of the inlet cap 43.
[0051] In the exhaust system as seen from ahead of the vehicle body
in FIG. 6, the exhaust pipe 20 includes a side curved portion 20A
which extends downward from the upper front surface of the cylinder
8 while curving outward on one side of the vehicle body. A
horizontally extended portion 20B extends from the lower end of the
side curved portion 20A ahead of the crankcase 9 in the transverse
direction of the vehicle body and bends back in an opposite
direction. The cylinder 8 and the crankcase 9 are schematically
shown.
[0052] The side curved portion 20A includes the forward curved
portion 40 and the rear curved portion (FIG. 2) and includes an
exhaust upstream portion of the extension 20a entering the inlet
cap 43 through the inlet 43a. The horizontally extended portion 20B
includes an exhaust downstream portion of the extension 20a and the
bend 48, and the rear portion 48c is in a position bending in an
opposite direction with respect to the extension 20a in a vertical
plane.
[0053] The extension 20a and the rear portion 48c are connected by
a spacer ring 49 to prevent vibration of the rear portion 48c. The
spacer ring 49 has a center hole 49a as shown in FIG. 4 and this
center hole 49a allows the inner spaces of the inlet cap 43, main
portion 44 and outlet cap 45 to communicate with each other. Hence,
exhaust gas flows through the following components in the following
order: exhaust pipe 20 to extension 20a to front portion 48a to
middle portion 48b to rear portion 48c to inlet cap 43 to main
portion 44 to outlet cap 45 to tail pipe 22.
[0054] The exhaust air which has passed through the extension 20a
and the bend 48 goes out through the rear portion 48c and expands
into the inner spaces of the inlet cap 43 and the main portion 44
and diffuses while further continuing to expand toward the outlet
cap 45, thereby reducing the exhaust air energy, then goes through
the outlet cap 45 with a tapered downstream portion to reach the
outlet 45a. During this process, the exhaust sound is sufficiently
reduced.
[0055] FIG. 7 is a sectional view taken along the line 7-7 in FIG.
5, where the rear end 22a of the tail pipe 22 has a double
structure with a band-like covering member 50 wound and welded
around the whole outer periphery and space 51 communicates with the
inside of the tail pipe 22 through a through hole 52 made in the
rear end 22a of the tail pipe 22. Therefore, the exhaust air again
expands into the space 51 at the outlet and the space 51 also
functions as a resonator so that the final exhaust sound level is
further decreased.
[0056] The functions of this embodiment will now be described. As
shown in FIG. 2, the muffler 21 is located ahead of the crankcase 9
and behind point P as the rear end of the front wheel 24 with the
front fork 23 constituting a front suspension in its most retracted
position and also above point R as the lowest end of the body frame
1. Therefore, the muffler 21, which is a relatively large-capacity
and is a large and heavy component, can be in a position nearer to
the head pipe 2 without interfering with the front wheel 24. Hence,
when the vehicle body moves centering around the head pipe 2, the
inertia force is reduced, which is suitable for an off-road
motorcycle.
[0057] In addition, the muffler 21 is located ahead of the
crankcase 9, namely the muffler is not located under the engine 7
which has an upright cylinder 8 and is longitudinally short.
Therefore, a high minimum road clearance is ensured, which is also
suitable as an exhaust structure for an off-road motorcycle.
[0058] Furthermore, the muffler 21 is located under the rear end of
the high front fender 24a located above the front wheel 24.
Therefore, the muffler can be in a position not to interfere with
the front fender 24a.
[0059] As shown in FIG. 4, the down frame 5 and the lower frame 6
constitute the front frame. Even though the vicinity of the joint
between the down frame 5 and the lower frame 6 is curved forward in
a convex pattern, the rear surface of the muffler is curved along
this curved portion, so that it can be in a position as near to the
front lower portion of the crankcase as possible.
[0060] The exhaust pipe 20, which extends forward and downward from
the cylinder head 11, includes an upper forward curved portion 40
and a lower backward curved portion 41, and the backward curved
portion 41 is connected with a rear lower portion of the muffler
21. Therefore, a sufficiently long exhaust pipe length is ensured
although the exhaust port of the cylinder head 11 is relatively
near to the muffler 21.
[0061] As shown in FIG. 6, the exhaust pipe 20 includes a side
curved portion 20A and a horizontally extended portion 20B and the
horizontally extended portion 20B is fitted in the muffler 21 and
is bent on the left and right with the downstream end 48d open into
the muffler 21. This ensures that that the exhaust pipe is
sufficiently long and can be arranged in a compact manner by taking
advantage of the inside of the large-capacity muffler extending
transversely.
[0062] The present invention is not limited to the above embodiment
and can be modified or applied in various forms within the
principle of the invention. For instance, it is also possible that
the extension 20a and the bend 48 are porous and a sound absorbing
material such as glass wool is wound around them to absorb sound.
It is also possible that a rear muffler separate from the muffler
21 is connected with the tail pipe 22. In this case, exhaust sound
is reduced more effectively; even in this case, the rear muffler
may be an auxiliary muffler, which is small and lightweight. One or
more such rear mufflers 1 may be provided.
[0063] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *